1. Field of the Invention
The present invention relates to a pilot-controlled electromagnetic directional control valve (pilot-controlled electromagnetic valve) for use in a pneumatically operated system.
2. Description of the Related Art
Japanese Patent Application Unexamined Publication (KOKAI) No. 2000-283118 (patent document 1) discloses a pilot-controlled electromagnetic valve in which a main valve chamber is formed at one side of a piston chamber, and a manual controller chamber is formed at the other side of the piston chamber with a partition interposed therebetween. A piston is slidably provided in the piston chamber. A main valve element is slidably provided in the main valve chamber. The main valve element is moved by the movement of the piston. FIG. 3 is a sectional view showing the pilot-controlled electromagnetic valve disclosed in the patent document 1, in which the disclosure of the patent document 1 is rearranged to clarify constituent members or portions common to the patent document 1 and the present invention.
In FIG. 3, a main valve block 10 is disposed adjacently to one side (right side in FIG. 3) of a manual controller-equipped piston block 11. The main valve block 10 and the piston block 11 are secured to each other by using a connecting member 20, etc. A pilot valve 12 is disposed adjacently to the other side (left side in FIG. 3) of the piston block 11. The piston block 11 and the pilot valve 12 are secured to each other by using bolts or the like. The main valve block 10 has a main valve chamber 13 extending therethrough in the longitudinal direction. A main valve element (spool) 14 is slidably provided in the main valve chamber 13. The main valve block 10 is provided with a first exhaust port EA, a first output port A, an air supply port P, a second output port B, and a second exhaust port EB that are open on the lower surface of the main valve block 10 in the order mentioned from one end thereof. Each port communicates with a predetermined portion of the main valve chamber 13.
The piston block 11 has a piston chamber 15 that is open at one side thereof. The piston chamber 15 has a larger diameter than that of the main valve chamber 13. The piston chamber 15 and the main valve chamber 13 are disposed concentrically and communicated with each other. A spring retainer 24 is secured to one end (right end in FIG. 3) of the main valve chamber 13 that communicates with the atmosphere. A spring 17 is interposed between the spring retainer 24 and the bottom of an opening 25 at one end of the main valve element 14. A piston 16 is slidably provided in the piston chamber 15. When compressed air is supplied into a piston first chamber (chamber at the left side of the piston 16 in FIG. 3) 22, the piston 16 and the main valve element 14 move in one direction (rightward in FIG. 3) against the force of the spring 17. It should be noted that two O-rings hermetically seal between the inner peripheral wall of the main valve chamber 13 and both end portions of the main valve element 14.
The piston block 11 is provided with a valve element 18, a supply port p, an output port a, and an exhaust port e. The supply port p communicates with the supply port P through a supply passage 27 in the piston block 11 and further through a supply passage 27a in the main valve block 10. The output port a communicates with the piston first chamber 22 through an output passage 28. The exhaust port e communicates with the atmosphere. The piston block 11 is formed with a manual controller chamber (not shown), and a manual controller (not shown) is provided in the manual controller chamber. In response to actuation of the manual controller, compressed air is supplied into the piston first chamber 22.
An annular groove is formed on the outer periphery of the piston 16. A piston packing 30 for sealing is fitted in the annular groove. In order to prevent air leakage through the piston packing 30 and non-uniform wear thereof, the piston 16 needs to be provided with a guide that prevents the piston 16 from tilting. Incidentally, the electromagnetic valve shown in FIG. 3 is of the two-position type. In the case of a three-position type electromagnetic valve, the guide is particularly important because there is an operating step at which the piston 16 and the main valve element 14 separate from each other. In general, a guide is provided at each side of the piston packing 30, as shown in FIG. 3. As the width of the guide increases, the overall length L of the electromagnetic valve increases, which goes against the demand for a reduction in size. Therefore, it is desired to eliminate the width L′ of the piston 16 to thereby shorten the piston 16. It is also desired to simplify the relationship between the manual controller and the piston 16.
Japanese Patent Application Unexamined Publication (KOKAI) No. 2002-250463 (patent document 2) discloses in FIG. 3, etc. thereof a pilot-controlled electromagnetic valve wherein an annular projection is provided on one side of the piston, and a small-diameter projection on the other side (i.e. side closer to the piston) of the main valve element is fitted to the annular projection. This structure is judged to be capable of preventing tilting of the piston and of eliminating the width L′ of the piston to thereby shorten the piston. Thus, the technique of preventing tilting of the piston and also shortening the piston has already been known. The present invention aims at preventing tilting of the piston and shortening the piston with a simplified arrangement.